International nuclear experts say Iran has linked up two more sets of centrifuges at its Natanz nuclear plant, this time underground.

The experts, who asked not to be identified, are with the International Atomic Energy Agency. They say Iran plans on testing the new sets soon, at first without using uranium.

Iran already has two sets, or cascades, of 164 centrifuges linked together above ground at its desert complex in Natanz. Tehran has announced plans to install 3,000 underground centrifuges in coming months, which would allow it to enrich uranium on a much wider and faster scale.

U.S. State Department spokesman, Sean McCormack said Monday that Tehran appears to be continuing what he called its "path of isolation." The spokesman added that the U.S. will push for "incremental" U.N. Security Council sanctions against Iran if it does not comply with U.N. demands to stop its uranium enrichment program.

The United States and other Western nations accuse Iran of seeking to build nuclear weapons. Iran denies the charge, saying its nuclear program is for civilian, non-military purposes.

Some information for this report was provided by AFP, AP and Reuters.